AM79C978A Datasheet, PDF (69/256 Pages) Advanced Micro Devices – Single-Chip 1/10 Mbps PCI Home Networking Controller

Table 11. Receive Address Match
PAM
0
1
0
0
0
LAFM
0
0
1
1
0
BAM
0
0
0
0
1
DRCVBC
X
X
0
1
0
Comment
Frame accepted
due to PROM = 1
Physical address
match
Logical address
filter match;
frame is not of
type broadcast
Logical address
filter match;
frame can be of
type broadcast
Broadcast frame
Automatic Pad Stripping
During reception of an IEEE 802.3 frame, the pad field
can be stripped automatically. Setting ASTRP_RCV
(CSR4, bit 0) to 1 enables the automatic pad stripping
feature. The pad field will be stripped before the frame
is passed to the FIFO, thus preserving FIFO space for
additional frames. The FCS field will also be stripped,
since it is computed at the transmitting station based on
the data and pad field characters, and will be invalid for
a receive frame that has had the pad characters
stripped.
The number of bytes to be stripped is calculated from the
embedded length field (as defined in the ISO 8802-3
(IEEE/ANSI 802.3) definition) contained in the frame.
The length indicates the actual number of LLC data bytes
contained in the message. Any received frame which
contains a length field less than 46 bytes will have the
pad field stripped (if ASTRP_RCV is set). Receive frames
which have a length field of 46 bytes or greater will be
passed to the host unmodified.
Figure 36 shows the byte/bit ordering of the received
length field for an IEEE 802.3-compatible frame format.
Since any valid Ethernet Type field value will always be
greater than a normal IEEE 802.3 Length field (≥46),
the Am79C978A controller will not attempt to strip valid
Ethernet frames. Note that for some network protocols,
the value passed in the Ethernet Type and/or IEEE
802.3 Length field is not compliant with either standard
and may cause problems if pad stripping is enabled.
Receive FCS Checking
Reception and checking of the received FCS is per-
formed automatically by the Am79C978A controller.
Note that if the Automatic Pad Stripping feature is en-
abled, the FCS for padded frames will be verified
against the value computed for the incoming bit stream
including pad characters, but the FCS value for a pad-
ded frame will not be passed to the host. If an FCS
error is detected in any frame, the error will be reported
in the CRC bit in RMD1.
Receive Exception Conditions
Exception conditions for frame reception fall into
two distinct categories, i.e., those conditions which
are the result of normal network operation, and
those which occur due to abnormal network and/or
host related events.
Normal events which may occur and which are handled
autonomously by the Am79C978A controller are basi-
cally collisions within the slot time and automatic runt
packet rejection. The Am79C978A controller will
ensure that collisions that occur within 512 bit times
from the start of reception (excluding preamble) will be
automatically deleted from the receive FIFO with no
host intervention.
Am79C978A
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